Magnetic tape with particular refractive index characteristics, magnetic tape cartridge, and magnetic tape apparatus

ABSTRACT

Provided are a magnetic tape including: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which a total thickness of the magnetic tape is equal to or smaller than 5.30 μm, the magnetic layer has a servo pattern, a center line average surface roughness Ra measured on a surface of the magnetic layer is equal to or smaller than 1.8 nm, and an absolute value ΔN of a difference between a refractive index Nxy of the magnetic layer, measured in an in-plane direction and a refractive index Nz of the magnetic layer, measured in a thickness direction is 0.25 to 0.40, a magnetic tape cartridge and a magnetic tape apparatus including this magnetic tape, a magnetic tape cartridge and a magnetic tape apparatus including this magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/727,181filed Dec. 26, 2019, which claims priority under 35 U.S.C 119 toJapanese Patent Application No. 2018-246871 filed on Dec. 28, 2018. Theabove applications are hereby expressly incorporated by reference, intheir entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tapecartridge, and a magnetic tape apparatus.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recordingmedia and disk-shaped magnetic recording media, and tape-shaped magneticrecording media, that is, magnetic tapes (hereinafter, simply referredto as “tapes”) are mainly used for data storage such as data back-up andarchive. The recording of information on a magnetic tape is normallyperformed by recording a magnetic signal on a data band of the magnetictape. Accordingly, data tracks are formed in the data band.

An increase in recording capacity (high capacity) of the magnetic tapeis required in accordance with a great increase in information contentin recent years. As means for realizing high capacity, a technique ofdisposing a larger amount of data tracks in a width direction of themagnetic tape by narrowing the width of the data track to increaserecording density is used.

However, in a case where the width of the data track is narrowed and therecording and/or reproducing of information is performed by allowing therunning of the magnetic tape in a magnetic tape apparatus (normallyreferred to as a “drive”), it is difficult that a magnetic headcorrectly follows the data tracks due to the position change of themagnetic tape, and errors may easily occur at the time of recordingand/or reproducing. Thus, as means for preventing occurrence of sucherrors, a system using a head tracking servo using a servo signal(hereinafter, referred to as a “servo system”) has been recentlyproposed and practically used (for example, see U.S. Pat. No.5,689,384A).

SUMMARY OF THE INVENTION

In a magnetic servo type servo system among the servo systems, a servopattern (servo signal) is formed in a magnetic layer of a magnetic tape,and this servo pattern is magnetically read to perform head tracking.More specific description is as follows.

First, a servo head reads a servo pattern formed in a magnetic layer(that is, reproduces a servo signal). A position of a magnetic head in amagnetic tape apparatus is controlled in accordance with a valueobtained by reading the servo pattern. Accordingly, in a case oftransporting the magnetic tape in the magnetic tape apparatus forrecording and/or reproducing information, it is possible to increase anaccuracy of the magnetic head following the data track, even in a casewhere the position of the magnetic tape is changed. For example, even ina case where the position of the magnetic tape is changed in the widthdirection with respect to the magnetic head, in a case of recordingand/or reproducing information by transporting the magnetic tape in themagnetic tape apparatus, it is possible to control the position of themagnetic head in the width direction of the magnetic tape in themagnetic tape apparatus, by performing the head tracking servo. By doingso, it is possible to correctly record information on the magnetic tapeand/or correctly reproduce information recorded on the magnetic tape inthe magnetic tape apparatus.

By the way, a magnetic tape is generally accommodated in a magnetic tapecartridge, circulated, and used. In order to increase recording capacityfor one reel of the magnetic tape cartridge, it is desirable to increasea total length of the magnetic tape accommodated in one reel of themagnetic tape cartridge. In order to increase the total length of themagnetic tape, it is necessary that a total thickness of the magnetictape is thinned (hereinafter, also referred to as “thinning”).

In recent years, the magnetic tape is required to increase the surfacesmoothness of the magnetic layer. Increasing the surface smoothness ofthe magnetic layer leads to the improvement of electromagneticconversion characteristics.

In view of the point described above, the present inventor studiedapplication of the magnetic tape with the thinned total thickness andthe increased surface smoothness of the magnetic layer to the servosystem. However, in the studies, it was clear that, in a case where thetotal thickness of the magnetic tape is thinned and the surfacesmoothness of the magnetic layer is increased, a phenomenon which hasnot been known in the related art occurs in which an occurrencefrequency of a signal defect increases at the time of reproducing theservo signal in the servo system. As an example of such a signal defect,a signal defect called thermal asperity is used. The thermal asperity isfluctuation in a reproduced waveform caused by change of a resistancevalue of a magnetoresistive (MR) element due to occurrence of a localtemperature change in the MR element in a system comprising an MR headon which the MR element is mounted. In a case where the signal defectoccurs at the time of reproducing the servo signal, it is difficult toperform head tracking at the occurrence location. Accordingly, it isrequired to reduce the occurrence frequency of the signal defect at thetime of reproducing the servo signal, in order to more correctly recordinformation on the magnetic tape and/or more correctly reproduceinformation recorded on the magnetic tape by using the servo system.

According to an aspect of the invention, an object is to reduce anoccurrence frequency of a signal defect in a servo system in a magnetictape with a thinned total thickness and an increased surface smoothnessof a magnetic layer.

According to an aspect of the invention, there is provided a magnetictape comprising: a non-magnetic support; and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support, inwhich a total thickness of the magnetic tape is equal to or smaller than5.30 the magnetic layer has a servo pattern, a center line averagesurface roughness Ra measured on a surface of the magnetic layer(hereinafter, also referred to as a “magnetic layer surface roughnessRa”) is equal to or smaller than 1.8 nm, and an absolute value ΔN(hereinafter, also referred to as “ΔN (of the magnetic layer)”) of adifference between a refractive index Nxy of the magnetic layer,measured in an in-plane direction and a refractive index Nz of themagnetic layer, measured in a thickness direction is 0.25 to 0.40.

In an aspect, Nxy>Nz may be satisfied and the difference (Nxy−Nz)between the refractive index Nxy and the refractive index Nz may be 0.25to 0.40.

In an aspect, the magnetic tape may further comprise a non-magneticlayer including non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.

In an aspect, the total thickness of the magnetic tape may be 3.00 μm to5.30 μm.

In an aspect, the magnetic tape may further comprise a back coatinglayer including non-magnetic powder and a binding agent on a surface ofthe non-magnetic support opposite to a surface provided with themagnetic layer.

In an aspect, the magnetic layer surface roughness Ra may be 1.2 nm to1.8 nm.

In an aspect, the servo pattern may be a timing-based servo pattern.

According to another aspect of the invention, there is provided amagnetic tape cartridge comprising: the magnetic tape described above.

According to still another aspect of the invention, there is provided amagnetic tape apparatus comprising: the magnetic tape described above;and a magnetic head.

According to an aspect of the invention, it is possible to provide amagnetic tape which is thinned, has a servo pattern in a magnetic layerwith high surface smoothness, and has a reduced occurrence frequency ofa signal defect at the time of reproducing a servo signal in a servosystem, and a magnetic tape cartridge and a magnetic tape apparatusincluding the magnetic tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of disposition of data bands and servo bands.

FIG. 2 shows a servo pattern disposition example of a linear-tape-open(LTO) Ultrium format tape.

FIG. 3 is a schematic illustration of an embodiment of a magnetic tapeof the invention. The magnetic tape has a non-magnetic support, amagnetic layer including ferromagnetic powder and a binding agent on thenon-magnetic support, a non-magnetic layer including non-magnetic powderand a binding agent between the non-magnetic support and the magneticlayer, and a back coating layer including non-magnetic powder and abinding agent on the surface of the non-magnetic support opposite to thesurface provided with the magnetic layer.

FIG. 4 illustrates a generalized depiction of how the refractive indexNxy of the magnetic layer, measured in an in-plane direction, and therefractive index Nz of the magnetic layer, measured in a thicknessdirection, are measured. The incidence rays and the Nx, Ny and Nzdirections are shown relative to the magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

An aspect of the invention relates to a magnetic tape including anon-magnetic support; and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, in which a totalthickness of the magnetic tape is equal to or smaller than 5.30 μm, themagnetic layer has a servo pattern, a magnetic layer surface roughnessRa is equal to or smaller than 1.8 nm, and ΔN of the magnetic layer is0.25 to 0.40.

Hereinafter, the magnetic tape will be described more specifically. Thefollowing description includes a surmise of the inventor. The inventionis not limited to such a surmise. In addition, hereinafter, exemplarydescription may be made with reference to the drawings. However, theinvention is not limited to the exemplified aspects.

Magnetic Layer

Magnetic Layer Surface Roughness Ra

A center line average surface roughness Ra (magnetic layer surfaceroughness Ra) measured on a surface of the magnetic layer of themagnetic tape is equal to or smaller than 1.8 nm. In the magnetic tapehaving the magnetic layer surface roughness Ra of equal to or smallerthan 1.8 nm and the total thickness of equal to or smaller than 5.30 μm,in a case where no measures are taken, an occurrence frequency of asignal defect increases at the time of reproducing a servo signal in aservo system. With respect to this, in the magnetic tape having ΔN ofthe magnetic layer of 0.25 to 0.40, it is possible to prevent occurrenceof the signal defect at the time of reproducing the servo signal inspite of the magnetic layer surface roughness Ra of equal to or smallerthan 1.8 nm and the total thickness of equal to or smaller than 5.30 μm.The surmise of the inventor regarding this point will be describedlater. In addition, the magnetic tape having the magnetic layer surfaceroughness Ra of equal to or smaller than 1.8 nm can exhibit excellentelectromagnetic conversion characteristics. From a viewpoint of furtherimproving the electromagnetic conversion characteristics, the magneticlayer surface roughness Ra is preferably equal to or smaller than 1.7 nmand more preferably equal to or smaller than 1.6 nm. In addition, themagnetic layer surface roughness Ra can be, for example, equal to orgreater than 1.2 nm or equal to or greater than 1.3 nm. However, fromthe viewpoint of improving the electromagnetic conversioncharacteristics, since the value of the magnetic layer surface roughnessRa is preferably as small as possible, it may be less than the valueexemplified above. In the invention and the specification, the “surface(of) the magnetic layer” of the magnetic tape has the same meaning asthe surface of the magnetic tape on the magnetic layer side.

The center line average surface roughness Ra measured on the surface ofthe magnetic layer of the magnetic tape in the invention and thespecification is a value measured in an area of 40 μm×40 μm of thesurface of the magnetic layer by an atomic force microscope (AFM). As anexample of measurement conditions, the following measurement conditionscan be used. The magnetic layer surface roughness Ra shown in exampleswhich will be described later is a value measured under the followingmeasurement conditions.

A region having the area of 40 μm×40 μm of the surface of the magneticlayer of the magnetic tape is measured by using the AFM (Nanoscope 4manufactured by Veeco Instruments Inc.) in a tapping mode. RTESP-300manufactured by Bruker Japan K.K. is used as a probe, a scan speed(probe moving speed) is 40 μm/sec, and a resolution is 512 pixels×512pixels.

The magnetic layer surface roughness Ra can be controlled by awell-known method. For example, the magnetic layer surface roughness Racan vary depending on sizes of various powder included in the magneticlayer (for example, ferromagnetic powder, non-magnetic powder which canbe randomly obtained therein, or the like), manufacturing conditions ofthe magnetic tape, and the like. Accordingly, the magnetic tape havingthe magnetic layer surface roughness Ra of equal to or smaller than 1.8nm can be obtained by adjusting these.

Servo Pattern

The magnetic tape includes a servo pattern in the magnetic layer. Theformation of the servo pattern on the magnetic layer is performed bymagnetizing a specific position of the magnetic layer by a servo writehead. A shape of the servo pattern and disposition thereof in themagnetic layer for realizing the head tracking servo are well known. Inregards to the servo pattern of the magnetic layer of the magnetic tape,a well-known technique can be used. For example, as a head trackingservo system, a timing-based servo system and an amplitude-based servosystem are known. The servo pattern of the magnetic layer of themagnetic tape may be a servo pattern capable of allowing head trackingservo of any system. In addition, a servo pattern capable of allowinghead tracking servo in the timing-based servo system and a servo patterncapable of allowing head tracking servo in the amplitude-based servosystem may be formed in the magnetic layer.

Hereinafter, as one specific aspect of the head tracking servo, headtracking servo in the timing-based servo system will be described.However, the head tracking servo in the timing-based servo system of theinvention is not limited to the following specific aspect.

In the head tracking servo in the timing-based servo system(hereinafter, referred to as “timing-based servo”), a plurality of servopatterns having two or more different shapes are formed on a magneticlayer, and a position of a servo head is recognized by an interval oftime in a case where the servo head has read two servo patterns havingdifferent shapes and an interval of time in a case where the servo headhas read two servo patterns having the same shapes. The position of themagnetic head in the width direction of the magnetic tape is controlledbased on the position of the servo head recognized as described above.In an aspect, the magnetic head, the position of which is controlledhere, is a magnetic head (reproducing head) which reproduces informationrecorded on the magnetic tape, and in another aspect, the magnetic headis a magnetic head (recording head) which records information in themagnetic tape.

FIG. 1 shows an example of disposition of data bands and servo bands. InFIG. 1, a plurality of servo bands 10 are disposed to be interposedbetween guide bands 12 in a magnetic layer of a magnetic tape 1. Aplurality of regions 11 each of which is interposed between two servobands are data bands. The servo pattern is a magnetized region and isformed by magnetizing a specific region of the magnetic layer by a servowrite head. The region magnetized by the servo write head (positionwhere a servo pattern is formed) is determined by standards. Forexample, in an LTO Ultrium format tape which is based on a localstandard, a plurality of servo patterns tilted in a tape width directionas shown in FIG. 2 are formed on a servo band, in a case ofmanufacturing a magnetic tape. Specifically, in FIG. 2, a servo frame SFon the servo band 10 is configured with a servo sub-frame 1 (SSF1) and aservo sub-frame 2 (SSF2). The servo sub-frame 1 is configured with an Aburst (in FIG. 2, reference numeral A) and a B burst (in FIG. 2,reference numeral B). The A burst is configured with servo patterns A1to A5 and the B burst is configured with servo patterns B1 to B5.Meanwhile, the servo sub-frame 2 is configured with a C burst (in FIG.2, reference numeral C) and a D burst (in FIG. 2, reference numeral D).The C burst is configured with servo patterns Cl to C4 and the D burstis configured with servo patterns D1 to D4. Such 18 servo patterns aredisposed in the sub-frames in the arrangement of 5, 5, 4, 4, as the setsof 5 servo patterns and 4 servo patterns, and are used for recognizingthe servo frames. FIG. 2 shows one servo frame for explaining. However,in practice, in the magnetic layer of the magnetic tape in which thehead tracking servo in the timing-based servo system is performed, aplurality of servo frames are disposed in each servo band in a runningdirection. In FIG. 2, an arrow shows the running direction. For example,an LTO Ultrium format tape generally includes 5,000 or more servo framesper a tape length of 1 m, in each servo band of the magnetic layer. Theservo head sequentially reads the servo patterns in the plurality ofservo frames, while coming into contact with and sliding on the surfaceof the magnetic layer of the magnetic tape transported in the magnetictape apparatus.

In the head tracking servo in the timing-based servo system, a positionof a servo head is recognized based on an interval of time in a casewhere the servo head has read the two servo patterns (reproduced servosignals) having different shapes and an interval of time in a case wherethe servo head has read two servo patterns having the same shapes. Theinterval of time is normally obtained as an interval of time of a peakof a reproduced waveform of a servo signal. For example, in the aspectshown in FIG. 2, the servo pattern of the A burst and the servo patternof the C burst are servo patterns having the same shapes, and the servopattern of the B burst and the servo pattern of the D burst are servopatterns having the same shapes. The servo pattern of the A burst andthe servo pattern of the C burst are servo patterns having the shapesdifferent from the shapes of the servo pattern of the B burst and theservo pattern of the D burst. An interval of the time in a case wherethe two servo patterns having different shapes are read by the servohead is, for example, an interval between the time in a case where anyservo pattern of the A burst is read and the time in a case where anyservo pattern of the B burst is read. An interval of the time in a casewhere the two servo patterns having the same shapes are read by theservo head is, for example, an interval between the time in a case whereany servo pattern of the A burst is read and the time in a case whereany servo pattern of the C burst is read. The head tracking servo in thetiming-based servo system is a system supposing that occurrence of adeviation of the interval of time is due to a position change of themagnetic tape in the width direction, in a case where the interval oftime is deviated from the set value. The set value is an interval oftime in a case where the magnetic tape runs without occurring theposition change in the width direction. In the timing-based servosystem, the magnetic head is moved in the width direction in accordancewith a degree of the deviation of the obtained interval of time from theset value. Specifically, as the interval of time is greatly deviatedfrom the set value, the magnetic head is greatly moved in the widthdirection. This point is applied to not only the aspect shown in FIGS. 1and 2, but also to entire timing-based servo systems.

For example, in a magnetic tape apparatus using the timing-based servosystem, in a case where the signal defect occurs at the time ofreproducing the servo signal, it is difficult to obtain measurementresults of the interval of time at a location (servo frame) where thedefect occurs. As a result, it is partially difficult to position thehead by moving the magnetic head in the width direction in a case ofrecording or reproducing a magnetic signal (information) by a magnetichead by allowing the magnetic tape to run. The occurrence of the signaldefect at the time of reproducing the servo signal in the magnetic tapeapparatus using the servo system as well as the timing-based servosystem makes it difficult to position the head by moving the magnetichead in a case of recording or reproducing a magnetic signal(information) by the magnetic head by allowing the magnetic tape to run.

In regards to the point described above, in the studies of the inventor,it was found that, in the magnetic tape having the total thickness ofequal to or smaller than 5.30 μm and the magnetic layer surfaceroughness Ra of equal to or smaller than 1.8 nm, the signal defectsignificantly occurs at the time of reproducing the servo signal. Theinventor has considered that a reason of the occurrence of the signaldefect at the time of reproducing the servo signal is that a smoothsliding between the servo head and the surface of the magnetic layer ishindered (hereinafter, referred to as a “decrease in sliding”). Theinventor has surmised that a reason of the decrease in sliding is thatthe magnetic tape having the total thickness of equal to or smaller than5.30 μm and the magnetic layer surface roughness Ra of equal to orsmaller than 1.8 nm is different from the magnetic tape of the relatedart in a contact state between the servo head and the surface of themagnetic layer. However, it is merely a surmise.

As a result of the intensive studies of the inventor with respect tothis, it was clear that such occurrence of the signal defect at the timeof reproducing the servo signal can be prevented by setting ΔN of themagnetic layer to be 0.25 to 0.40. The surmise of the inventor regardingthis point will be described later.

ΔN of Magnetic Layer

In the invention and the specification, the absolute value ΔN of thedifference between the refractive index Nxy of the magnetic layer,measured in an in-plane direction and the refractive index Nz of themagnetic layer, measured in a thickness direction is a value obtained bythe following method.

A refractive index of the magnetic layer in each direction is obtainedby a spectral ellipsometry using a double-layer model. In order toobtain a refractive index of the magnetic layer by a spectralellipsometry using a double-layer model, a value of a refractive indexof a portion adjacent to the magnetic layer is used. Hereinafter, anexample in a case of obtaining the refractive indexes Nxy and Nz of themagnetic layer in a magnetic tape having a layer configuration in whicha non-magnetic layer and a magnetic layer are laminated on anon-magnetic support in this order will be described. However, themagnetic tape according to an aspect of the invention may also be amagnetic tape having a layer configuration in which a magnetic layer isdirectly laminated on a non-magnetic support without the non-magneticlayer interposed therebetween. Regarding the magnetic tape having such aconfiguration, a refractive index of the magnetic layer in eachdirection is obtained in the same manner as the following method, usinga double-layer model of a magnetic layer and a non-magnetic support.Moreover, an incidence angle shown below is an incidence angle in a casewhere an incidence angle is 0° in a case of normal incidence.

(1) Preparation of Sample for Measurement

Regarding a magnetic tape including a back coating layer on a surface ofa non-magnetic support on a side opposite to the surface provided with amagnetic layer, the measurement is performed after removing the backcoating layer of a sample for measurement cut from the magnetic tape.The removal of the back coating layer can be performed by a well-knownmethod of dissolving of the back coating layer using a solvent or thelike. As the solvent, for example, methyl ethyl ketone can be used.However, any solvent which can remove the back coating layer may beused. The surface of the non-magnetic support after removing the backcoating layer is roughened by a well-known method so that the reflectedlight on this surface is not detected, in the measurement ofellipsometer. The roughening can be performed by a well-known methodsuch as polishing the surface of the non-magnetic support after removingthe back coating layer by using sand paper, for example. Regarding thesample for measurement cut out from the magnetic tape not including theback coating layer, the surface of the non-magnetic support on a sideopposite to the surface provided with the magnetic layer is roughened.

In addition, in order to measure the refractive index of thenon-magnetic layer described below, the magnetic layer is furtherremoved and the surface of the non-magnetic layer is exposed. In orderto measure the refractive index of the non-magnetic support describedbelow, the non-magnetic layer is also further removed and the surface ofthe non-magnetic support on the magnetic layer side is exposed. Theremoval of each layer can be performed by a well-known method so asdescribed regarding the removal of the back coating layer. Alongitudinal direction described below is a direction which was alongitudinal direction of the magnetic tape, in a case where the samplefor measurement is included in the magnetic tape before being cut out.This point applies to other directions described below, in the samemanner.

(2) Measurement of Refractive Index of Magnetic Layer

Δ (phase difference of s-polarized light and p-polarized light) and Ψ(amplitude ratio of s-polarized light and p-polarized light) aremeasured, using an ellipsometer, by setting an incidence angle to 65°,70°, and 75° and irradiating a magnetic layer surface with an incidenceray having a beam diameter of 300 μm in a longitudinal direction. Themeasurement is performed by changing a wavelength of an incidence ray byevery 1.5 nm in a range of 400 to 700 nm, and a measurement value ateach wavelength is obtained.

The refractive index of the magnetic layer at each wavelength isobtained with a double-layer model as described below, by using themeasurement values of Δ and Ψ of the magnetic layer at each wavelength,the refractive index of the non-magnetic layer in each directionobtained by the following method, and the thickness of the magneticlayer.

The zeroth layer which is a substrate of the double-layer model is setas a non-magnetic layer and the first layer thereof is set as a magneticlayer. The double-layer model is created by assuming that there is noeffect of rear surface reflection of the non-magnetic layer, by onlyconsidering the reflection of the interfaces of air/magnetic layer andmagnetic layer/non-magnetic layer. A refractive index of the first layerthat most closely matches the obtained measurement value is obtained byfitting the measurement value by a least squares method. A refractiveindex Nx of the magnetic layer in a longitudinal direction and arefractive index Nz₁ of the magnetic layer in a thickness directionmeasured by emitting an incidence ray in a longitudinal direction areobtained as values at the wavelength of 600 nm obtained from the resultsof the fitting.

In the same manner as described above, except that a direction in whichan incidence ray is incident is set as a width direction of the magnetictape, a refractive index Ny of the magnetic layer in a width directionand a refractive index Nz₂ of the magnetic layer in a thicknessdirection measured by emitting an incidence ray in a width direction areobtained as values at the wavelength of 600 nm obtained from the resultsof the fitting.

Fitting is performed by the following method.

In general, “complex refractive index n=η+ix”. Here, η is a real numberof the refractive index, κ is an extinction coefficient, and i is animaginary number. In a case where a complex dielectric constant ε=ε1+1ε2(ε1 and ε2 satisfies Kramers-Kronig relation), ε1=η²−κ², and ε2=2ηκ, thecomplex dielectric constant of Nx satisfies that ε_(x)=ε_(x)1+iε_(x)2,and the complex dielectric constant of Nz₁ satisfies thatε_(z1)1=ε_(z1)1+iε_(z1)2, in a case of calculating the Nx and Nz₁.

The Nx is obtained by setting ε_(x)2 as one Gaussian, setting any point,where a peak position is 5.8 to 5.1 eV and σ is 4 to 3.5 eV, as astarting point, setting a parameter to be offset to a dielectricconstant beyond a measurement wavelength range (400 to 700 nm), andperforming least squares fitting with respect to the measurement value.In the same manner, Nz₁ is obtained by setting any point of ε_(z1)2,where a peak position is 3.2 to 2.9 eV and σ is 1.5 to 1.2 eV, as astarting point, and setting an offset parameter, and performing leastsquares fitting with respect to the measurement value. Ny and Nz₂ arealso obtained in the same manner. The refractive index Nxy, measured inan in-plane direction of the magnetic layer is obtained as“Nxy=(Nx+Ny)/2”. The refractive index Nz of the magnetic layer, measuredin a thickness direction is obtained as “Nz=(Nz₁+Nz₂)/2”. From theobtained Nxy and Nz, the absolute value ΔN of difference thereof isobtained.

(3) Measurement of Refractive Index of Non-Magnetic Layer

Refractive indexes of the non-magnetic layer at a wavelength of 600 nm(the refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in a thicknessdirection measured by emitting an incidence ray in a longitudinaldirection, and the refractive index in a thickness direction measured byemitting an incidence ray in a width direction) are obtained in the samemanner as in the method described above, except the following points.

A wavelength of an incidence ray is changed by every 1.5 nm in the rangeof 250 to 700 nm.

By using a double-layer model of a non-magnetic layer and a non-magneticsupport, the zeroth layer which is a substrate of the double-layer modelis set as the non-magnetic support, and the first layer thereof is setas the non-magnetic layer. The double-layer model is created by assumingthat there is no effect of rear surface reflection of the non-magneticsupport, by only considering the reflection of the interfaces ofair/non-magnetic layer and non-magnetic layer/non-magnetic support.

In the fitting, seven peaks (0.6 eV, 2.3 eV, 2.9 eV, 3.6 eV, 4.6 eV, 5.0eV, and 6.0 eV) are assumed in the imaginary part (ε2) of the complexdielectric constant, and the parameter to be offset is set to thedielectric constant beyond the measurement wavelength range (250 to 700nm).

(4) Measurement of Refractive Index of Non-Magnetic Support

The refractive indexes of the non-magnetic support at a wavelength of600 nm (refractive index in the longitudinal direction, the refractiveindex in the width direction, the refractive index in a thicknessdirection measured by emitting an incidence ray in a longitudinaldirection, and the refractive index in a thickness direction measured byemitting an incidence ray in a width direction) used for obtaining therefractive indexes of the non-magnetic layer by the double-layer modelare obtained in the same manner as in the method described above formeasuring the refractive index of the magnetic layer, except thefollowing points.

A single-layer model with only front surface reflection is used, withoutusing the double-layer model.

Fitting is performed by a Cauchy model (n=A+B/λ², n is a refractiveindex, A and B are respectively constants determined by fitting, and λis a wavelength).

The occurrence of the signal defect at the time of reproducing the servosignal in the magnetic tape having the total thickness of equal to orsmaller than 5.30 μm and the magnetic layer surface roughness Ra ofequal to or smaller than 1.8 nm can be prevented by setting ΔN of themagnetic layer obtained by the method described above to be 0.25 to0.40. The occurrence of the signal defect is considered to be caused bythe decrease in sliding between the servo head and the surface of themagnetic layer. On the other hand, the inventor has considered that ΔNobtained by the method described above is a value which may be an indexof a presence state of the ferromagnetic powder in a surface region ofthe magnetic layer. This ΔN is surmised as a value which is influencedby the effect of various factors such as a presence state of a bindingagent or a density distribution of the ferromagnetic powder, in additionto the alignment state of the ferromagnetic powder in the magneticlayer. In addition, it is considered that the magnetic layer in whichthe ΔN is set as 0.25 to 0.40 by controlling various factors has a highhardness of the surface of the magnetic layer and the chipping thereofhardly occurs even under the sliding with the servo head. As a result,the inventor has surmised that the prevention of the attachment of thescraps generated due to the chipping of the surface of the magneticlayer as a foreign material onto the servo head contributes to theprevention of the decrease in sliding between the servo head and thesurface of the magnetic layer. The inventor has considered that thisleads to the prevention of the occurrence of the signal defect. However,this is merely a surmise and the invention is not limited to thesurmise.

ΔN of the magnetic layer of the magnetic tape is 0.25 to 0.40. From aviewpoint of further preventing the occurrence of the signal defect, ΔNis preferably 0.25 to 0.35. A specific aspect of means for adjusting ΔNwill be described later.

ΔN is an absolute value of a difference between Nxy and Nz. Nxy is arefractive index of the magnetic layer, measured in an in-planedirection and Nz is a refractive index of the magnetic layer, measuredin a thickness direction. In an aspect, a relation of Nxy>Nz can besatisfied, and in the other aspect, Nxy<Nz can be satisfied. From aviewpoint of electromagnetic conversion characteristics of the magnetictape, a relationship of Nxy>Nz is preferable, and therefore, thedifference between the Nxy and Nz (Nxy−Nz) is preferably 0.25 to 0.40and more preferably 0.25 to 0.35. In an aspect, Nxy can be, for example,1.50 to 2.50. In an aspect, Nz can be, for example, 1.30 to 2.50.However, the magnetic tape may have ΔN of 0.25 to 0.40, and Nxy and Nzare not limited to the ranges exemplified above.

Various means for adjusting ΔN described above will be described later.

Next, the magnetic layer will be described in detail.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic recordingmedium. From this viewpoint, ferromagnetic powder having an averageparticle size equal to or smaller than 50 nm is preferably used, andferromagnetic powder having an average particle size equal to or smallerthan 40 nm is more preferably used, as the ferromagnetic powder. On theother hand, from a viewpoint of stability of magnetization, the averageparticle size of the ferromagnetic powder is preferably equal to orgreater than 5 nm, more preferably equal to or greater than 10 nm, andeven more preferably equal to or greater than 15 nm.

As a preferred specific example of the ferromagnetic powder, hexagonalferrite powder can be used. The hexagonal ferrite powder can be bariumferrite, strontium ferrite, calcium ferrite, lead ferrite, or the like,or may be a mixed crystal of two or more kinds of these. For details ofthe hexagonal ferrite powder, descriptions disclosed in paragraphs 0012to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A,paragraphs 0013 to 0030 of JP2012-204726A, and paragraphs 0029 to 0084of JP2015-127985A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, metalpowder can also be used. For details of the metal powder, descriptionsdisclosed in paragraphs 0137 to 0141 of JP2011-216149A and paragraphs0009 to 0023 of JP2005-251351A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, ε-ironoxide powder can also be used. As a manufacturing method of the s-ironoxide powder, a manufacturing method from a goethite, a reverse micellemethod, and the like are known. All of the manufacturing methods arewell known. Regarding a method of manufacturing the ε-iron oxide powderin which a part of Fe is substituted with substitutional atoms such asGa, Co, Ti, Al, or Rh, a description disclosed in J. Jpn. Soc. PowderMetallurgy Vol. 61 Supplement, No. 51, pp. S280 to S284, J. Mater. Chem.C, 2013, 1, pp. 5200 to 5206 can be referred, for example. However, themanufacturing method of the ε-iron oxide powder capable of being used asthe ferromagnetic powder in the magnetic layer is not limited.

In the invention and the specification, “ferromagnetic powder” means anaggregate of a plurality of ferromagnetic particles. The “aggregation”includes not only an aspect in which the particles constituting theaggregation are in direct contact with each other, but also an aspect inwhich a binding agent, an additive, and the like are interposed betweenthe particles is also included. The point described above applies tovarious powder such as the non-magnetic powder in the invention and thespecification in the same manner.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder are values measured bythe following method with a transmission electron microscope, unlessotherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so as to have the total magnification ratio of 500,000 toobtain an image of particles constituting the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetic mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The value regardingthe size of the powder such as the average particle size shown inexamples which will be described later is a value measured by usingtransmission electron microscope H-9000 manufactured by Hitachi, Ltd. asthe transmission electron microscope, and image analysis software KS-400manufactured by Carl Zeiss as the image analysis software, unlessotherwise noted.

As a method of collecting sample powder from the magnetic tape in orderto measure the particle size, a method disclosed in a paragraph 0015 ofJP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles constituting thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetic mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

In an aspect, the shape of the ferromagnetic particles constituting theferromagnetic powder included in the magnetic layer can be a plateshape. Hereinafter, the ferromagnetic powder including the plate-shapedferromagnetic particles is referred to as a plate-shaped ferromagneticpowder. An average plate ratio of the plate-shaped ferromagnetic powdercan be preferably 2.5 to 5.0. The average plate ratio is an arithmeticmean of (maximum long diameter/thickness or height) in a case of thedefinition (2). As the average plate ratio increases, uniformity of thealignment state of the ferromagnetic particles constituting theplate-shaped ferromagnetic powder tends to easily increase by thealignment process, and the value of ΔN tends to increase.

As an index for a particle size of the ferromagnetic powder, anactivation volume can be used. The “activation volume” is a unit ofmagnetization reversal. Regarding the activation volume described in theinvention and the specification, magnetic field sweep rates of acoercivity Hc measurement part at time points of 3 minutes and 30minutes are measured by using a vibrating sample magnetometer in anenvironment of an atmosphere temperature 23° C.±1° C., and theactivation volume is a value acquired from the following relationalexpression of Hc and an activation volume V. The activation volume shownin examples which will be described later is a value acquired byperforming the measurement using a vibrating sample magnetometermanufactured by Toei Industry Co., Ltd.

Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturationmagnetization, k: Boltzmann's constant, T: absolute temperature, V:activation volume, A: spin precession frequency, and t: magnetic fieldreversal time]

From a viewpoint of improvement of recording density, the activationvolume of the ferromagnetic powder is preferably equal to or smallerthan 2,500 nm³, more preferably equal to or smaller than 2,300 nm³, andeven more preferably equal to or smaller than 2,000 nm³. On the otherhand, from a viewpoint of stability of magnetization, the activationvolume of the ferromagnetic powder is, for example, preferably equal toor greater than 800 nm³, more preferably equal to or greater than 1,000nm³, and even more preferably equal to or greater than 1,200 nm³.

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. The components other than the ferromagnetic powder ofthe magnetic layer are at least a binding agent, and one or more kindsof additives may be randomly included. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement of recording density.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape and includes a bindingagent in the magnetic layer. The binding agent is one or more kinds ofresin. The resin may be a homopolymer or a copolymer. As the bindingagent included in the magnetic layer, a resin selected from apolyurethane resin, a polyester resin, a polyamide resin, a vinylchloride resin, an acrylic resin obtained by copolymerizing styrene,acrylonitrile, or methyl methacrylate, a cellulose resin such asnitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylalresin such as polyvinyl acetal or polyvinyl butyral can be used alone ora plurality of resins can be mixed with each other to be used. Amongthese, a polyurethane resin, an acrylic resin, a cellulose resin, and avinyl chloride resin are preferable. These resins can be used as thebinding agent even in the non-magnetic layer and/or a back coating layerwhich will be described later. For the binding agent described above,description disclosed in paragraphs 0029 to 0031 of JP2010-024113A canbe referred to. In addition, the binding agent may be a radiationcurable resin such as an electron beam curable resin. For the radiationcurable resin, a description disclosed in paragraphs 0044 and 0045 ofJP2011-048878A can be referred to.

An average molecular weight of the resin used as the binding agent canbe, for example, 10,000 to 200,000 as a weight-average molecular weight.The weight-average molecular weight of the invention and thespecification is a value obtained by performing polystyrene conversionof a value measured by gel permeation chromatography (GPC). Asmeasurement conditions, the following conditions can be used. Theweight-average molecular weight shown in examples which will bedescribed later is a value obtained by performing polystyrene conversionof a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In an aspect, as the binding agent, a binding agent including an acidicgroup can be used. The acidic group of the invention and thespecification is used as a meaning including a form of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to be dissociated into anions and a salt thereof. As specific examplesof the acidic group, a form of a sulfonic acid group, a sulfuric acidgroup, a carboxy group, a phosphoric acid group, and a salt thereof canbe used. For example, a form of a salt of a sulfonic acid group (—SO₃H)means a group represented by —SO₃M, where M represents a grouprepresenting an atom (for example, alkali metal atom or the like) whichmay be cations in water or in an aqueous solvent. The same applies tothe form of each of salts of the various groups described above. As anexample of the binding agent including the acidic group, a resinincluding at least one kind of acidic group selected from the groupconsisting of a sulfonic acid group and a salt thereof (for example, apolyurethane resin or a vinyl chloride resin) can be used. However, theresin included in the magnetic layer is not limited to these resins. Inaddition, in the binding agent including the acidic group, a content ofthe acidic group can be, for example, 20 to 500 eq/ton. The unit “eq”represents equivalent, and is a unit that cannot be converted into an SIunit. The content of various functional groups such as the acidic groupincluded in the resin can be obtained by a well-known method inaccordance with the kind of the functional group. As the binding agenthaving a great content of the acidic group is used, the value of ΔNtends to increase. The amount of the binding agent used in a magneticlayer forming composition can be, for example, 1.0 to 30.0 parts bymass, and preferably 1.0 to 20.0 parts by mass with respect to 100.0parts by mass of the ferromagnetic powder. As the amount of the bindingagent used with respect to the ferromagnetic powder increases, the valueof ΔN tends to increase.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in anaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point also applies to a layer formedby using a composition, in a case where the composition used for formingthe other layer includes the curing agent. The preferred curing agent isa thermosetting compound, polyisocyanate being suitable. For details ofthe polyisocyanate, descriptions disclosed in paragraphs 0124 and 0125of JP2011-216149A can be referred to. The amount of the curing agent canbe, for example, 0 to 80.0 parts by mass with respect to 100.0 parts bymass of the binding agent in the magnetic layer forming composition, andis preferably 50.0 to 80.0 parts by mass, from a viewpoint ofimprovement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and a binding agent,and may include one or more kinds of additives, if necessary. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude non-magnetic powder, a lubricant, a dispersing agent, adispersing assistant, an antibacterial agent, an antistatic agent, anantioxidant, and carbon black. As the additives, a commerciallyavailable product can be suitably selected and used according to thedesired properties. For example, for the lubricant, a descriptiondisclosed in paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817Acan be referred to. The non-magnetic layer may include the lubricant.For the lubricant which may be included in the non-magnetic layer, adescription disclosed in paragraphs 0030, 0031, 0034, 0035, and 0036 ofJP2016-126817A can be referred to. For the dispersing agent, adescription disclosed in paragraphs 0061 and 0071 of JP2012-133837A canbe referred to. The dispersing agent may be included in the non-magneticlayer. For the dispersing agent which may be included in thenon-magnetic layer, a description disclosed in a paragraph 0061 ofJP2012-133837A can be referred to.

The magnetic layer preferably includes one kind or two or more kinds ofnon-magnetic powders. As the non-magnetic powder, non-magnetic powder(hereinafter, referred to as a “projection formation agent”) which canfunction as a projection formation agent which forms projectionssuitably protruded from the surface of the magnetic layer can be used.The projection formation agent is a component which can contribute tocontrol of friction properties of the surface of the magnetic layer ofthe magnetic tape. In addition, the magnetic layer may includenon-magnetic powder (hereinafter, referred to as an “abrasive”) whichcan function as an abrasive. The magnetic layer of the magnetic tapepreferably includes at least one of the projection formation agent orthe abrasive and more preferably includes both of them.

As the projection formation agent, various non-magnetic powders normallyused as a projection formation agent can be used. These may be powder ofan inorganic substance or powder of an organic substance. In an aspect,from a viewpoint of homogenization of friction properties, particle sizedistribution of the projection formation agent is not polydispersionhaving a plurality of peaks in the distribution and is preferablymonodisperse showing a single peak. From a viewpoint of availability ofmonodisperse particles, the non-magnetic powder included in the magneticlayer is preferably powder of inorganic substances (inorganic powder).Examples of the inorganic powder include powder of inorganic oxide suchas metal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide, and powder of inorganic oxide is preferable.The projection formation agent is more preferably colloidal particlesand even more preferably inorganic oxide colloidal particles. Inaddition, from a viewpoint of availability of monodisperse particles,the inorganic oxide colloidal particles are more preferably colloidalsilica (silica colloidal particles). In the invention and thespecification, the “colloidal particles” are particles which are notprecipitated and dispersed to generate a colloidal dispersion, in a casewhere 1 g of the particles is added to 100 mL of at least one organicsolvent of at least methyl ethyl ketone, cyclohexanone, toluene, orethyl acetate, or a mixed solvent including two or more kinds of thesolvent described above at any mixing ratio. The average particle sizeof the colloidal particles is a value obtained by a method disclosed ina paragraph 0015 of JP2011-048878A as a measurement method of an averageparticle diameter. In addition, in another aspect, the projectionformation agent is preferably carbon black.

An average particle size of the projection formation agent is, forexample, 30 to 300 nm and is preferably 40 to 200 nm.

Meanwhile, the abrasive is preferably non-magnetic powder having Mohshardness exceeding 8 and more preferably non-magnetic powder having Mohshardness equal to or greater than 9. A maximum value of Mohs hardness is10 of diamond. Specifically, powders of alumina (Al₂O₃), siliconcarbide, boron carbide (B₄C), SiO₂, TiC, chromium oxide (Cr₂O₃), ceriumoxide, zirconium oxide (ZrO₂), iron oxide, diamond, and the like can beused, and among these, alumina powder such as α-alumina and siliconcarbide powder are preferable. Regarding the particle size of theabrasive, a specific surface area which is an index of a particle sizeis, for example, equal to or greater than 14 m²/g, preferably equal toor greater than 16 m²/g, and more preferably equal to or greater than 18m²/g. In addition, the specific surface area of the abrasive can be, forexample, equal to or smaller than 40 m²/g. The specific surface area isa value obtained by a nitrogen adsorption method (also referred to as aBrunauer-Emmett-Teller (BET) one-point method). Hereinafter, thespecific surface area obtained by such a method is also referred to as aBET specific surface area.

In addition, from a viewpoint that the projection formation agent andthe abrasive can exhibit each function in more excellent manner, thecontent of the projection formation agent of the magnetic layer ispreferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.Meanwhile, the content of the abrasive in the magnetic layer ispreferably 1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 partsby mass, and even more preferably 4.0 to 10.0 parts by mass with respectto 100.0 parts by mass of the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive of the magnetic layer formingcomposition.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on the non-magnetic support or mayinclude a non-magnetic layer including the non-magnetic powder and thebinding agent between the non-magnetic support and the magnetic layer.The non-magnetic powder used in the non-magnetic layer may be powder ofinorganic substances or powder of organic substances. In addition,carbon black and the like can be used. Examples of the inorganicsubstance include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. The non-magnetic powdercan be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black which can be used in the non-magnetic layer,descriptions disclosed in paragraphs 0040 and 0041 of JP2010-024113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50% to 90% by mass andmore preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technique regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technique regarding the magneticlayer can be applied.

The non-magnetic layer of the magnetic tape also includes asubstantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, and aromatic polyamidesubjected to biaxial stretching are used. Among these, polyethyleneterephthalate, polyethylene naphthalate, and polyamide are preferable.Corona discharge, plasma treatment, easy-bonding treatment, or heattreatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer includingnon-magnetic powder and a binding agent on a surface of the non-magneticsupport opposite to the surface provided with the magnetic layer. Theback coating layer preferably includes any one or both of carbon blackand inorganic powder. In regards to the binding agent included in theback coating layer and various additives which can be randomly includedtherein, a well-known technique regarding the back coating layer can beapplied, and a well-known technique regarding the list of the magneticlayer and/or the non-magnetic layer can also be applied. For example,for the back coating layer, descriptions disclosed in paragraphs 0018 to0020 of JP2006-331625A and page 4, line 65 to page 5, line 38, of U.S.Pat. No. 7,029,774B can be referred to.

Various Thicknesses

The total thickness of the magnetic tape is equal to or smaller than5.30 μm. A thin total thickness (thinning) is preferable for increasinga recording capacity for one reel of the magnetic tape cartridge. Thetotal thickness of the magnetic tape, for example, may be equal to orsmaller than 5.20 μm, equal to or smaller than 5.10 μm, or equal to orsmaller than 5.00 μm. In addition, the total thickness of the magnetictape, for example, is preferably equal to or greater than 1.00 μm, morepreferably equal to or greater than 2.00 μm, even more preferably equalto or greater than 3.00 μm, and still even more preferably equal to orgreater than 4.00 μm from a viewpoint of ease of handling(handleability) of the magnetic tape.

A thickness of the non-magnetic support of the magnetic tape ispreferably 3.00 to 4.50 μm. From a viewpoint of high-density recordingrequired in recent years, a thickness of the magnetic layer ispreferably equal to or smaller than 0.15 μm and more preferably equal toor smaller than 0.10 μm. A thickness of the magnetic layer is even morepreferably 0.01 to 0.10 μm. The magnetic layer may be at least onelayer, or the magnetic layer can be separated into two or more layershaving magnetic properties, and a configuration regarding a well-knownmultilayered magnetic layer can be applied. A thickness of the magneticlayer which is separated into two or more layers is a total thickness ofthe layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and more preferably 0.10 to 0.70 μm.

The thicknesses of various layers and the non-magnetic support areobtained by exposing a cross section of the magnetic tape in a thicknessdirection by a well-known method of ion beams or microtome, andobserving the exposed cross section with a scanning transmissionelectron microscope (STEM). For the specific examples of the measurementmethod of the thickness, a description disclosed regarding themeasurement method of the thickness in examples which will be describedlater can be referred to.

Manufacturing Step

Preparation of Each Layer Forming Composition

Steps of preparing the composition for forming the magnetic layer, thenon-magnetic layer, or the back coating layer generally include at leasta kneading step, a dispersing step, and a mixing step provided beforeand after these steps, if necessary. Each step may be divided into twoor more stages. The components used in the preparation of each layerforming composition may be added at an initial stage or in a middlestage of each step. As the solvent, one kind or two or more kinds ofvarious solvents generally used for manufacturing a coating typemagnetic recording medium can be used. For the solvent, a descriptiondisclosed in a paragraph 0153 of JP2011-216149A can be referred to, forexample. In addition, each component may be separately added in two ormore steps. For example, the binding agent may be separately added inthe kneading step, the dispersing step, and a mixing step for adjustinga viscosity after the dispersion. In order to manufacture the magnetictape, a well-known manufacturing technique of the related art can beused in various steps. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to.As a disperser, a well-known disperser can be used. In addition, theferromagnetic powder and the abrasive can also be dispersed separately.The separate dispersion is specifically a method of preparing a magneticlayer forming composition through a step of mixing an abrasive solutionincluding an abrasive and a solvent (here, ferromagnetic powder is notsubstantially included) with a magnetic liquid including theferromagnetic powder, a solvent, and a binding agent. The expression“ferromagnetic powder is not substantially included” means that theferromagnetic powder is not added as a constituent component of theabrasive solution, and a small amount of the ferromagnetic powder mixedas impurities without any intention is allowed. Regarding ΔN, as aperiod of the dispersion time of the magnetic liquid increases, thevalue of ΔN tends to increase. This is considered that, as a period ofthe dispersion time of the magnetic liquid increases, the dispersibilityof the ferromagnetic powder in the coating layer of the magnetic layerforming composition increases, and the uniformity of the alignment stateof the ferromagnetic particles configuring the ferromagnetic powder bythe alignment process tends to easily increase. In addition, as theperiod of the dispersion time in a case of mixing and dispersing variouscomponents of the non-magnetic layer forming composition increases, thevalue of ΔN tends to increase. The dispersion time of the magneticliquid and the dispersion time of the non-magnetic layer formingcomposition may be set so that ΔN of 0.25 to 0.40 can be realized.

In any stage of preparing each layer forming composition, the filteringmay be performed by a well-known method. The filtering can be performedby using a filter, for example. As the filter used in the filtering, afilter having a hole diameter of 0.01 to 3 μm (for example, filter madeof glass fiber or filter made of polypropylene) can be used, forexample.

Coating Step

The magnetic layer can be formed, for example, by directly applying themagnetic layer forming composition onto the non-magnetic support orperforming multilayer coating of the magnetic layer forming compositionwith the non-magnetic layer forming composition in order or at the sametime. The back coating layer can be formed by applying the back coatinglayer forming composition onto a side of the non-magnetic supportopposite to the side provided with the magnetic layer (or magnetic layeris to be provided). In addition, the coating step for forming each layercan be also performed by being divided into two or more steps. Forexample, in an aspect, the magnetic layer forming composition can beapplied in two or more steps. In this case, a drying process may beperformed or may not be performed during the coating steps of twostages. In addition, the alignment process may be performed or may notbe performed during the coating steps of two stages. For details of thecoating for forming each layer, a description disclosed in a paragraph0066 of JP2010-231843A can be referred to. In addition, for the dryingstep after applying the each layer forming composition, a well-knowntechnique can be applied. Regarding the magnetic layer formingcomposition, as a drying temperature of a coating layer which is formedby applying the magnetic layer forming composition (hereinafter, alsoreferred to as a “coating layer of the magnetic layer formingcomposition” or simply a “coating layer”) decreases, the value of ΔNtends to increase. The drying temperature can be an atmospheretemperature for performing the drying step, for example, and may be setso that ΔN of 0.25 to 0.40 can be realized.

Other Steps

For various other steps for manufacturing the magnetic tape,descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A canbe referred to.

For example, it is preferable to perform the alignment process withrespect to the coating layer of the magnetic layer forming compositionwhile the coating layer is wet. From a viewpoint of ease of exhibitingof ΔN of 0.25 to 0.40, the alignment process is preferably performed bydisposing a magnet so that a magnetic field is vertically applied to thesurface of the coating layer of the magnetic layer forming composition(that is, homeotropic alignment process). The strength of the magneticfield during the alignment process may be set so that ΔN of 0.25 to 0.40can be realized. In addition, in a case of performing the coating stepof the magnetic layer forming composition by the coating steps of two ormore stages, it is preferable to perform the alignment process at leastafter the final coating step, and it is more preferable to perform thehomeotropic alignment process. For example, in a case of forming themagnetic layer by the coating steps of two stages, the drying step isperformed without performing the alignment process after the firstcoating step, and then, the alignment process can be performed withrespect to the formed coating layer in the second coating step. For thealignment process, various well-known techniques such as descriptionsdisclosed in a paragraph 0052 of JP2010-024113A can be used. Forexample, the homeotropic alignment process can be performed by awell-known method such as a method using a pole opposing magnet. In thealignment zone, a drying speed of the coating layer can be controlleddepending on a temperature and an air flow of dry air and/or atransportation speed of the magnetic tape in the alignment zone. Inaddition, the coating layer may be preliminarily dried before thetransportation to the alignment zone.

In addition, the calender process can be performed in any stage afterdrying the coating layer of the magnetic layer forming composition. Forthe conditions of the calender process, a description disclosed in aparagraph 0026 of JP2010-231843A can be referred to. As the calendertemperature (surface temperature of the calender roll) increases, thevalue of ΔN tends to increase. As the calender temperature increases,the value of the magnetic layer surface roughness Ra tends to increase.The calender temperature may be set so that ΔN of 0.25 to 0.40 and Ra ofequal to or smaller than 1.8 nm can be realized.

Formation of Servo Pattern

The magnetic tape includes a servo pattern in the magnetic layer.Details of the servo pattern are as described above. For example, FIG. 1shows an example of disposition of a region (servo band) in which thetiming-based servo pattern is formed and a region (data band) interposedbetween two servo bands. FIG. 2 shows an example of disposition of thetiming-based servo patterns. Here, the example of disposition shown ineach drawing is merely an example, and the servo pattern, the servobands, and the data bands may be disposed in the disposition accordingto a system of the magnetic tape apparatus (drive). In addition, for theshape and the disposition of the timing-based servo pattern, awell-known technique such as examples of disposition shown in FIG. 4,FIG. 5, FIG. 6, FIG. 9, FIG. 17, and FIG. 20 of U.S. Pat. No. 5,689,384Acan be applied, for example.

The servo pattern can be formed by magnetizing a specific region of themagnetic layer by a servo write head mounted on a servo writer. Thedirection of magnetization can be the longitudinal direction or avertical direction (in other words, an in-plane direction) of themagnetic tape. The formation of the servo pattern is normally formedafter direct current (DC) demagnetization of the magnetic layer. Thedirection of demagnetization can be the longitudinal direction or thevertical direction of the magnetic tape. The region magnetized by theservo write head (position where a servo pattern is formed) isdetermined by standards. As the servo writer, a commercially availableservo writer or a servo writer having a well-known configuration can beused. For the configuration of the servo writer, well-known techniquessuch as techniques disclosed in JP2011-175687A, U.S. Pat. Nos.5,689,384A, and 6,542,325B can be referred to.

As described above, it is possible to obtain the magnetic tape accordingto an aspect of the invention. The magnetic tape is normallyaccommodated in a magnetic tape cartridge and the magnetic tapecartridge is mounted on a magnetic tape apparatus.

Magnetic Tape Cartridge

An aspect of the invention relates to a magnetic tape cartridgeincluding the magnetic tape.

In the magnetic tape cartridge, the magnetic tape is generallyaccommodated in a cartridge main body in a state of being wound around areel. The reel is rotatably provided in the cartridge main body. As themagnetic tape cartridge, a single reel type magnetic tape cartridgeincluding one reel in a cartridge main body and a twin reel typemagnetic tape cartridge including two reels in a cartridge main body arewidely used. In a case where the single reel type magnetic tapecartridge is mounted in the magnetic tape apparatus (drive) in order torecord and/or reproduce information (magnetic signals) on the magnetictape, the magnetic tape is drawn from the magnetic tape cartridge andwound around the reel on the drive side. A magnetic head is disposed ona magnetic tape transportation path from the magnetic tape cartridge toa winding reel. Sending and winding of the magnetic tape are performedbetween a reel (supply reel) on the magnetic tape cartridge side and areel (winding reel) on the drive side. In the meantime, the magnetichead comes into contact with and slides on the surface of the magneticlayer of the magnetic tape, and accordingly, the recording and/orreproduction of information is performed. With respect to this, in thetwin reel type magnetic tape cartridge, both reels of the supply reeland the winding reel are provided in the magnetic tape cartridge. Themagnetic tape cartridge may be any of single reel type magnetic tapecartridge and twin reel type magnetic tape cartridge. The magnetic tapecartridge may include the magnetic tape according to an aspect of theinvention, and a well-known technique can be applied for otherconfigurations.

Magnetic Tape Apparatus

An aspect of the invention relates to a magnetic tape apparatusincluding the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic tape apparatus”means a device capable of performing at least one of the recording ofinformation on the magnetic tape or the reproducing of informationrecorded on the magnetic tape. Such an apparatus is generally called adrive. The magnetic tape apparatus can be a sliding type magnetic tapeapparatus. The sliding type apparatus is an apparatus in which thesurface of the magnetic layer comes into contact with and slides on themagnetic head, in a case of performing the recording of information onthe magnetic tape and/or reproducing of the recorded information.

The magnetic head included in the magnetic tape apparatus can be arecording head capable of performing the recording of information on themagnetic tape, or can be a reproducing head capable of performing thereproducing of information recorded on the magnetic tape. In addition,in an aspect, the magnetic tape apparatus can include both of arecording head and a reproducing head as separate magnetic heads. Inanother aspect, the magnetic head included in the magnetic tape can alsohave a configuration of comprising both of a recording element and areproducing element in one magnetic head. As the reproducing head, amagnetic head (MR head) including a magnetoresistive (MR) elementcapable of sensitively reading information recorded on the magnetic tapeas a reproducing element is preferable. As the MR head, variouswell-known MR heads can be used. In addition, the magnetic head whichperforms the recording of information and/or the reproducing ofinformation may include a servo pattern reading element. Alternatively,as a head other than the magnetic head which performs the recording ofinformation and/or the reproducing of information, a magnetic head(servo head) comprising a servo pattern reading element may be includedin the magnetic tape apparatus.

The details of the magnetic tape mounted on the magnetic tape apparatusare as described above. Such a magnetic tape includes servo patterns.Accordingly, a magnetic signal is recorded on the data band by themagnetic head to form a data track, and/or, in a case of reproducing therecorded signal, head tracking is performed based on the read servopattern, while reading the servo pattern by the servo head, andtherefore, it is possible to cause the magnetic head to follow the datatrack at a high accuracy.

For details of the head tracking servo of the timing-based servo system,for example, well-known techniques such as techniques disclosed in U.S.Pat. Nos. 5,689,384A, 6,542,325B, and U.S. Pat. No. 7,876,521B can beused. In addition, for the details of the head tracking servo in theamplitude-based servo system, well-known techniques disclosed in U.S.Pat. Nos. 5,426,543A and 5,898,533A can be used.

A commercially available magnetic tape apparatus generally includes amagnetic head in accordance to a standard. In addition, a commerciallyavailable magnetic tape apparatus generally has a servo controllingmechanism for realizing head tracking of the servo system in accordanceto a standard. The magnetic tape apparatus according to an aspect of theinvention can be configured by incorporating the magnetic tape accordingto an aspect of the invention to a commercially available magnetic tapeapparatus.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and “%by mass”, unless otherwise noted. In addition, steps and evaluationsdescribed below are performed in an environment of an atmospheretemperature of 23° C.±1° C., unless otherwise noted.

Example 1

Preparation of Abrasive Solution 3.0 parts of 2,3-dihydroxynaphthalene(manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32%solution (solvent is a mixed solvent of methyl ethyl ketone and toluene)of a SO₃Na group-containing polyester polyurethane resin (UR-4800 (SO₃Nagroup: 0.08 meq/g) manufactured by Toyobo Co., Ltd.), and 570.0 parts ofa mixed solvent of methyl ethyl ketone and cyclohexanone (mass ratio of1:1) as a solvent were mixed with 100.0 parts of alumina powder (HIT-80manufactured by Sumitomo Chemical Co., Ltd.) having a gelatinizationratio of approximately 65% and a BET specific surface area of 20 m²/g,and dispersed in the presence of zirconia beads by a paint shaker for 5hours. After the dispersion, the dispersion liquid and the beads wereseparated by a mesh and an alumina dispersion was obtained.

Preparation of Magnetic Layer Forming Composition

Magnetic Liquid Plate-shaped ferromagnetic hexagonal 100.0 parts bariumferrite powder activation volume: 1,600 nm³, average plate ratio: 3.5SO₃Na group-containing polyurethane resin see Table 1 Weight-averagemolecular weight: 70,000, SO₃Na group: see Table 1 Cyclohexanone 150.0parts Methyl ethyl ketone 150.0 parts Abrasive Solution Aluminadispersion prepared as described above 6.0 parts Silica Sol (projectionforming agent liquid) Colloidal silica (average particle size: 100 nm)2.0 parts Methyl ethyl ketone 1.4 parts Other Components Stearic acid2.0 parts Butyl stearate 2.0 parts Polyisocyanate (CORONATE (registeredtrademark) 2.5 parts manufactured by Tosoh Corporation) FinishingAdditive Solvent Cyclohexanone 200.0 parts Methyl ethyl ketone 200.0parts

Preparation Method

The magnetic liquid was prepared by beads-dispersing of variouscomponents of the magnetic liquid described above by using beads as thedispersion medium in a batch type vertical sand mill. The beaddispersion was performed using zirconia beads (bead diameter: seeTable 1) as the beads for the time shown in Table 1 (magnetic liquidbead dispersion time).

The magnetic liquid obtained as described above, the abrasive solution,silica sol, other components, and a finishing additive solvent weremixed with each other and beads-dispersed for 5 minutes, and thetreatment (ultrasonic dispersion) was performed with a batch typeultrasonic device (20 kHz, 300 W) for 0.5 minutes. After that, theobtained mixed solution was filtered by using a filter having a holediameter of 0.5 μm, and the magnetic layer forming composition wasprepared.

Preparation of Non-Magnetic Layer Forming Composition

Each component among various components of the non-magnetic layerforming composition shown below excluding stearic acid, butyl stearate,cyclohexanone, and methyl ethyl ketone was beads-dispersed (dispersionmedium: zirconia beads (bead diameter: 0.1 mm), dispersion time: seeTable 1) by using a batch type vertical sand mill to obtain a dispersionliquid. After that, the remaining components were added into theobtained dispersion liquid and stirred with a dissolver. Then, theobtained dispersion liquid was filtered with a filter (hole diameter:0.5 μm) and a non-magnetic layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide 100.0 parts Average particlesize (average major axis length): 0.15 μm Average acicular ratio: 7 BETspecific surface area: 52 m²/g Carbon black 20.0 parts Average particlesize: 20 nm Electron beam curable vinyl chloride copolymer 13.0 partsElectron beam curable polyurethane resin 6.0 parts Stearic acid 1.0 partButyl stearate 1.0 part Cyclohexanone 300.0 parts Methyl ethyl ketone300.0 parts

Preparation of Back Coating Layer Forming Composition

Each component among various components of the back coating layerforming composition shown below excluding stearic acid, butyl stearate,polyisocyanate, and cyclohexanone was kneaded and diluted by an openkneader, and a mixed solution was obtained. After that, the obtainedmixed solution was subjected to a dispersion process of 12 passes, witha transverse beads mill and zirconia beads having a bead diameter of 1.0mm, by setting a bead filling percentage as 80 volume %, acircumferential speed of rotor distal end as 10 m/sec, and a retentiontime for 1 pass as 2 minutes. After that, the remaining components wereadded into the obtained dispersion liquid and stirred with a dissolver.Then, the obtained dispersion liquid was filtered with a filter (holediameter: 1.0 nm) and a back coating layer forming composition wasprepared.

Non-magnetic inorganic powder: α-iron oxide 80.0 parts Average particlesize (average major axis length): 0.15 μm Average acicular ratio: 7 BETspecific surface area: 52 m²/g Carbon black 20.0 parts Average particlesize: 20 nm A vinyl chloride copolymer 13.0 parts A sulfonic acid saltgroup- 6.0 parts containing polyurethane resin Phenylphosphonic acid 3.0parts Methyl ethyl ketone 155.0 parts Stearic acid 3.0 parts Butylstearate 3.0 parts Polyisocyanate 5.0 parts Cyclohexanone 355.0 parts

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition was applied and dried onto apolyethylene naphthalate support, and then, an electron beam was emittedwith an energy of 40 kGy at an acceleration voltage of 125 kV, to form anon-magnetic layer.

The magnetic layer forming composition was applied on a surface of theformed non-magnetic layer to form a coating layer. A homeotropicalignment process and a drying process were performed by applying amagnetic field having a strength shown in a column of “formation andalignment of magnetic layer” of Table 1 to the surface of the coatinglayer in a vertical direction by using a pole opposing magnet in anatmosphere at an atmosphere temperature (magnetic layer dryingtemperature) shown in Table 1, while this coating layer was wet, and amagnetic layer was formed.

After that, the back coating layer forming composition was applied anddried on a surface of the support on a side opposite to the surface onwhich the non-magnetic layer and the magnetic layer were formed.

Then, a surface smoothing treatment (calender process) was performedwith a calender roll configured of only a metal roll, at a calenderprocess speed of 80 m/min, linear pressure of 300 kg/cm (294 kN/m), anda calender temperature (surface temperature of a calender roll) shown inTable 1.

Then, the thermal treatment was performed in the environment of theatmosphere temperature of 70° C. for 36 hours. After the thermaltreatment, the slitting was performed so as to have a width of ½ inches(1 inch is 0.0254 meters), and the surface of the magnetic layer wascleaned with a tape cleaning device in which a nonwoven fabric and arazor blade are attached to a device including a sending and windingdevice of the slit so as to press the surface of the magnetic layer.Then, servo patterns (timing-based servo patterns) having dispositionand shapes according to the LTO Ultrium format were formed on themagnetic layer by a commercially available servo writer.

As described above, the magnetic tape of Example 1 was manufactured.

Examples 2 and 4, Comparative Examples 1 to 4 and 6, and ReferenceExamples 1 to 4

A magnetic tape was manufactured by the same method as that in Example1, except that various conditions shown in Table 1 were changed as shownin Table 1. The thickness of each layer was adjusted by the coatingamount of the each layer forming composition.

In Table 1, in the comparative examples and the reference examples inwhich “no alignment process” is shown in the column of “formation andalignment of magnetic layer”, the magnetic tape was manufactured withoutperforming the alignment process regarding the coating layer of themagnetic layer forming composition.

Example 3

After forming the non-magnetic layer on the polyethylene naphthalatesupport in the same manner as in Example 1, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 50 nm to form a first coating layer.The first coating layer was passed through the atmosphere at theatmosphere temperature shown in Table 1 (magnetic layer dryingtemperature) without application of a magnetic field to form a firstmagnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 50 nm to form a second coating layer. A homeotropic alignmentprocess and a drying process were performed by applying a magnetic fieldhaving a strength shown in the column of “formation and alignment ofmagnetic layer” of Table 1 to the surface of the second coating layer ina vertical direction by using a pole opposing magnet in an atmosphere atan atmosphere temperature (magnetic layer drying temperature) shown inTable 1, while this second coating layer was wet, and a second magneticlayer was formed.

A magnetic tape was manufactured by the same method as that in Example1, except that the multilayered magnetic layer was formed as describedabove.

Comparative Example 5

After forming the non-magnetic layer on the polyethylene naphthalatesupport in the same manner as in Example 1, the magnetic layer formingcomposition was applied on the surface of the non-magnetic layer so thatthe thickness after drying becomes 50 nm to form a first coating layer.The homeotropic alignment process and the drying process were performedby applying a magnetic field having a strength shown in the column of“formation and alignment of magnetic layer” of Table 1 to the surface ofthe first coating layer in a vertical direction by using a pole opposingmagnet in an atmosphere at an atmosphere temperature (magnetic layerdrying temperature) shown in Table 1, while this first coating layer waswet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on thesurface of the first magnetic layer so that the thickness after dryingbecomes 50 nm to form a second coating layer. The second coating layerwas passed through the atmosphere at the atmosphere temperature shown inTable 1 (magnetic layer drying temperature) without application of amagnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured by the same method as that in Example1, except that the multilayered magnetic layer was formed as describedabove.

Measurement Method

(1) Magnetic Layer Surface Roughness Ra

A measurement area of 40 μm×40 μm on the surface of the magnetic layerof the magnetic tape was measured by using an atomic force microscope(AFM, Nanoscope 4 manufactured by Veeco Instruments Inc.) in a tappingmode, and a center line average surface roughness Ra (magnetic layersurface roughness Ra) was obtained. RTESP-300 manufactured by BrukerJapan K.K. was used as a probe, a scan speed (probe moving speed) was 40μm/sec, and a resolution was 512 pixels×512 pixels.

(2) Thicknesses of Non-Magnetic Support and Each Layer

The thicknesses of the magnetic layer, the non-magnetic layer, thenon-magnetic support, and the back coating layer of each manufacturedmagnetic tape were measured by the following method. Table 1 shows themeasured various thicknesses and the total magnetic tape thicknesscalculated from the various thicknesses.

The thicknesses of the magnetic layer, the non-magnetic layer, and thenon-magnetic support measured here were used for calculating thefollowing refractive index.

(i) Manufacturing of Cross Section Observation Sample

A cross section observation sample including the entire region in thethickness direction from the magnetic layer side surface of the magnetictape to the back coating layer side surface was manufactured accordingto the method disclosed in paragraphs 0193 and 0194 of JP2016-177851A.

(ii) Thickness Measurement

The manufactured sample was observed with the STEM and a STEM image wascaptured. This STEM image was a STEM-high-angle annular dark field(HAADF) image which is captured at an acceleration voltage of 300 kV anda magnification ratio of imaging of 450,000, and imaging was performedso that the entire region in the thickness direction from the magneticlayer side surface of the magnetic tape to the back coating layer sidesurface was included in one image. In the STEM image obtained asdescribed above, a linear line connecting both ends of a line segmentshowing the surface of the magnetic layer to each other was determinedas a reference line showing the surface of the magnetic tape on themagnetic layer side. In a case where the STEM image was captured so thatthe magnetic layer side of the cross section observation sample waspositioned on the upper side of the image and the back coating layerside was positioned on the lower side, for example, the linear lineconnecting both ends of the line segment described above to each otheris a linear line connecting an intersection between a left side of theimage (shape is a rectangular or square shape) of the STEM image and theline segment, and an intersection between a right side of the STEM imageand the line segment to each other. In the same manner as describedabove, a reference line showing the interface between the magnetic layerand the non-magnetic layer, a reference line showing the interfacebetween the non-magnetic layer and the non-magnetic support, a referenceline showing the interface between the non-magnetic support and the backcoating layer, and a reference line showing the surface of the magnetictape on the back coating layer side were determined.

The thickness of the magnetic layer was obtained as the shortestdistance from one position randomly selected on the reference lineshowing the surface of the magnetic tape on the magnetic layer side tothe reference line showing the interface between the magnetic layer andthe non-magnetic layer. In the same manner as described above, thethicknesses of the non-magnetic layer, the non-magnetic support, and theback coating layer were obtained.

(3) ΔN of Magnetic Layer

Hereinafter, M-2000U manufactured by J. A. Woollam Co. was used as anellipsometer. Creation and fitting of the double-layer model or thesingle-layer model were performed using WVASE32 manufactured by J. A.Woollam Co. as analysis software.

(i) Measurement Refractive Index of Non-Magnetic Support

A sample for measurement was cut out from each magnetic tape. The clothnot used was permeated with fresh methyl ethyl ketone, the back coatinglayer of the sample for measurement was wiped off and removed using thiscloth to expose the surface of the non-magnetic support, and then, thissurface is roughened with sand paper so that reflected light of theexposed surface is not detected in the measurement which will beperformed after this with an ellipsometer.

After that, by causing the cloth to permeate with fresh methyl ethylketone, by wiping off and removing the magnetic layer and thenon-magnetic layer of the sample for measurement using the cloth andbonding a surface of a silicon wafer and the roughened surface to eachother using static electricity, the sample for measurement was disposedon the silicon wafer so that the surface of the non-magnetic supportexposed by removing the magnetic layer and the non-magnetic layer(hereinafter, referred to as the “surface of the non-magnetic support onthe magnetic layer side”) faced upward.

An incidence ray was incident to the surface of the non-magnetic supportof the sample for measurement on the magnetic layer side on the siliconwafer using an ellipsometer as described above, to measure D and y. Byusing the obtained measurement values and the thickness of thenon-magnetic support obtained in the section (2), the refractive indexesof the non-magnetic support (the refractive index in a longitudinaldirection, the refractive index in a width direction, the refractiveindex in a thickness direction measured by incidence of incidence ray inthe longitudinal direction, and the refractive index in a thicknessdirection measured by incidence of incidence ray in the width direction)were obtained by the method described above.

(ii) Measurement of Refractive Index of Non-Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The clothnot used was permeated with methyl ethyl ketone, the back coating layerof the sample for measurement was wiped off and removed using this clothto expose the surface of the non-magnetic support, and then, thissurface is roughened with sand paper so that reflected light of theexposed surface is not detected in the measurement which will beperformed after this with the spectroscopic ellipsometer.

After that, the cloth not used was permeated with fresh methyl ethylketone, the surface of the magnetic layer of the sample for measurementwas wiped off using this cloth, the magnetic layer was removed to exposethe surface of the non-magnetic layer, and then, the sample formeasurement was disposed on the silicon wafer in the same manner as inthe section (i).

The measurement regarding the surface of the non-magnetic layer of thesample for measurement on the silicon wafer was performed using anellipsometer, and the refractive indexes of the non-magnetic layer (therefractive index in a longitudinal direction, the refractive index in awidth direction, the refractive index in a thickness direction measuredby incidence of incidence ray in the longitudinal direction, and therefractive index in a thickness direction measured by incidence ofincidence ray in the width direction) were obtained by the methoddescribed above in a spectral ellipsometry.

(iii) Measurement of Refractive Index of Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The clothnot used was permeated with fresh methyl ethyl ketone, the back coatinglayer of the sample for measurement was wiped off and removed using thiscloth to expose the surface of the non-magnetic support, and then, thissurface is roughened with sand paper so that reflected light of theexposed surface is not detected in the measurement which will beperformed after this with the spectroscopic ellipsometer.

After that, the sample for measurement was disposed on the sample formeasurement on the silicon wafer, in the same manner as in the section(i).

The measurement regarding the surface of the magnetic layer of thesample for measurement on the silicon wafer was performed using anellipsometer, and the refractive indexes of the magnetic layer (therefractive index Nx in a longitudinal direction, the refractive index Nyin a width direction, the refractive index Nz₁ in a thickness directionmeasured by incidence of incidence ray in the longitudinal direction,and the refractive index Nz₂ in a thickness direction measured byincidence of incidence ray in the width direction) were obtained by themethod described above in a spectral ellipsometry. Nxy and Nz wereobtained from the obtained values, and the absolute value ΔN of thedifference of these values was obtained. Regarding all of magnetic tapesof the examples, the comparative examples, and the reference examples,the obtained Nxy was a value greater than Nz (that is, Nxy>Nz).

(4) Vertical Squareness Ratio (SQ)

A vertical squareness ratio of the magnetic tape is a squareness ratiomeasured in a vertical direction of the magnetic tape. The “verticaldirection” described regarding the squareness ratio is a directionorthogonal to the surface of the magnetic layer. Regarding each magnetictape of the examples, the comparative examples, and the referenceexamples, the vertical squareness ratio was obtained by sweeping anexternal magnetic field in the magnetic tape at a measurementtemperature of 23° C.±1° C. using an vibrating sample magnetometer(manufactured by Toei Industry Co., Ltd.) under conditions of a maximumexternal magnetic field of 1194 kA/m (15 kOe) and a scan speed of 4.8kA/m/sec (60 Oe/sec). The measurement value is a value after diamagneticfield correction, and is obtained as a value obtained by subtractingmagnetization of a sample probe of the vibrating sample magnetometer asbackground noise. In an aspect, the vertical squareness ratio of themagnetic tape is preferably 0.60 to 1.00. In addition, in an aspect, thevertical squareness ratio of the magnetic tape can be, for example,equal to or smaller than 0.90, equal to or smaller than 0.85, or equalto or smaller than 0.80, and can also be greater than these values.

(5) Occurrence Frequency of Signal Defect (Thermal Asperity) at Time ofReproducing Servo Signal

The magnetic tapes of the examples, the comparative examples, and thereference examples were attached to a servo tester. In the servo tester,the reading of the servo patterns (reproducing of servo signals) wasperformed by the servo head by allowing each magnetic tape to run andthe surface of the magnetic layer of the running magnetic tape and theservo head on which the MR element is mounted to come into contact witheach other and sliding. In a reproduced waveform of the servo signalobtained by the reproduction, a part that is not a normal burst signaland shows an output of equal to or greater than 200% with an averagevalue of a noise level output as 100% is determined as thermal asperity,and the number of thermal asperities was counted. A value (number oftimes/m) obtained by dividing the counted number of thermal asperitiesby total length of the magnetic tape is defined as the occurrencefrequency of the signal defect (thermal asperity).

The results are shown in Table 1 (Table 1-1 to Table 1-4).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Magnetic layerthickness (μm) 0.10 0.10 0.10 0.10 Non-magnetic layer thickness 0.700.50 0.70 0.70 (gm) Non-magnetic support thickness 4.20 4.00 4.20 4.20(μm) Back coating layer thickness 0.30 0.30 0.30 0.30 (μm) Magnetic tapetotal thickness 5.30 4.90 5.30 5.30 (μm) Magnetic liquid bead dispersion50 hours 50 hours 50 hours 50 hours time Magnetic liquid dispersion bead0.1 mm 0.1 mm 0.1 mm 0.1 mm diameter Magnetic liquid 330 eq/ton 330eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethaneresin Magnetic liquid 15.0 parts 15.0 parts 15.0 parts 15.0 partsContent of SO₃Na group-containing polyurethane resin Non-magnetic layerforming 24 hours 24 hours 24 hours 24 hours composition dispersion timeMagnetic layer drying 50° C. 50° C. 50° C. 50° C. temperature Calendertemperature 100° C. 100° C. 100° C. 100° C. Formation and alignment ofHomeotropic Homeotropic Second magnetic Homeotropic magnetic layeralignment 0.5 T alignment 0.5 T layer: homeotropic alignment 0.2 Talignment 0.5 T/ First magnetic layer: no alignment process Magneticlayer surface 1.8  1.8  1.8  1.8  roughness Ra (nm) Vertical squarenessratio (SQ) 0.66 0.66 0.60 0.60 Nxy 1.90 1.90 1.95 1.90 Nz 1.60 1.60 1.601.65 ΔN 0.30 0.30 0.35 0.25 Occurrence frequency of signal 0.10 0.150.10 0.05 defect (thermal asperity) (number of times/m)

TABLE 1-2 Comparative Comparative Comparative Example 1 Example 2Example 3 Magnetic layer thickness (μm) 0.10 0.10 0.10 Non-magneticlayer thickness 0.70 0.50 0.50 (μm) Non-magnetic support thickness 4.204.00 4.00 (μm) Back coating layer thickness 0.30 0.30 0.30 (μm) Magnetictape total thickness 5.30 4.90 4.90 (μm) Magnetic liquid bead dispersion6 hours 6 hours 6 hours time Magnetic liquid dispersion bead 1.0 mm 1.0mm 1.0 mm diameter Magnetic liquid 60 eq/ton 60 eq/ton 60 eq/ton Contentof SO₃Na group of polyurethane resin Magnetic liquid 25.0 parts 25.0parts 25.0 parts Content of SO₃Na group-containing polyurethane resinNon-magnetic layer forming 3 hours 3 hours 3 hours compositiondispersion time Magnetic layer drying 70° C. 70° C. 70° C. temperatureCalender temperature 100° C. 100° C. 105° C. Formation and alignment ofNo alignment process No alignment process No alignment process magneticlayer Magnetic layer surface 1.8  1.8  1.6  roughness Ra (nm) Verticalsquareness ratio (SQ) 0.50 0.50 0.50 Nxy 1.90 1.90 1.90 Nz 1.80 1.801.80 ΔN 0.10 0.10 0.10 Occurrence frequency of signal 1.00 1.50 7.50defect (thermal asperity) (number of times/m)

TABLE 1-3 Comparative Comparative Comparative Example 4 Example 5Example 6 Magnetic layer thickness (μm) 0.10 0.10 0.10 Non-magneticlayer thickness 0.70 0.70 0.70 (μm) Non-magnetic support thickness 4.204.20 4.20 (μm) Back coating layer thickness 0.30 0.30 0.30 (μm) Magnetictape total thickness 5.30 5.30 5.30 (μm) Magnetic liquid bead dispersion50 hours 50 hours 96 hours time Magnetic liquid dispersion bead 0.1 mm0.1 mm 0.1 mm diameter Magnetic liquid 330 eq/ton 330 eq/ton 330 eq/tonContent of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts15.0 parts 10.0 parts Content of SO₃Na group-containing polyurethaneresin Non-magnetic layer forming 24 hours 24 hours 48 hours compositiondispersion time Magnetic layer drying 50° C. 50° C. 30° C. temperatureCalender temperature 100° C. 100° C. 110° C. Formation and alignment ofNo alignment process Second magnetic Homeotropic magnetic layer layer:alignment 0.5 T no alignment process/ First magnetic layer: homeotropicalignment 0.5 T Magnetic layer surface 1.8  1.8  1.5  roughness Ra (nm)Vertical squareness ratio (SQ) 0.53 0.60 0.80 Nxy 1.90 1.90 2.20 Nz 1.701.70 1.75 ΔN 0.20 0.20 0.45 Occurrence frequency of signal 8.00 10.00 13.00  defect (thermal asperity) (number of times/m)

TABLE 1-4 Reference Reference Reference Reference Example 1 Example 2Example 3 Example 4 Magnetic layer thickness (μm) 0.10 0.10 0.10 0.10Non-magnetic layer thickness 1.00 0.70 0.70 0.50 (μm) Non-magneticsupport thickness 4.30 4.20 4.20 4.00 (μm) Back coating layer thickness0.60 0.40 0.30 0.30 (μm) Magnetic tape total thickness 6.00 5.40 5.304.90 (μm) Magnetic liquid bead dispersion 6 hours 6 hours 6 hours 6hours time Magnetic liquid dispersion bead 1.0 mm 1.0 mm 1.0 mm 1.0 mmdiameter Magnetic liquid 60 eq/ton 60 eq/ton 60 eq/ton 60 eq/ton Contentof SO₃Na group of polyurethane resin Magnetic liquid 25.0 parts 25.0parts 25.0 parts 25.0 parts Content of SO₃Na group-containingpolyurethane resin Non-magnetic layer forming 3 hours 3 hours 3 hours 3hours composition dispersion time Magnetic layer drying 70° C. 70° C.70° C. 70° C. temperature Calender temperature 100° C. 100° C. 90° C.90° C. Formation and alignment of No alignment No alignment No alignmentNo alignment magnetic layer process process process process Magneticlayer surface 1.8  1.8  2.2  2.2  roughness Ra (nm) Vertical squarenessratio (SQ) 0.50 0.50 0.50 0.50 Nxy 1.90 1.90 1.90 1.90 Nz 1.80 1.80 1.801.80 ΔN 0.10 0.10 0.10 0.10 Occurrence frequency of signal 0.08 0.100.05 0.12 defect (thermal asperity) (number of times/m)

By comparison of Reference Examples 1 to 4 and Comparative Examples 1 to6 with each other, in a case where the total thickness of the magnetictape is equal to or smaller than 5.30 μm and the magnetic layer surfaceroughness Ra is equal to or smaller than 1.8 nm compared to a case wherethe total thickness of the magnetic tape is greater than 5.30 μm(Reference Examples 1 and 2) and a case where the magnetic layer surfaceroughness Ra is greater than 1.8 nm (Reference Examples 3 and 4), it wasconfirmed that the occurrence frequency of the signal defectsignificantly increases at the time of reproducing the servo signal(Comparative Examples 1 to 6).

With respect to this, the magnetic tapes of Examples 1 to 4 have thetotal thickness of equal to or smaller than 5.30 μm and the magneticlayer surface roughness Ra of equal to or smaller than 1.8 nm, but theoccurrence frequency of the signal defect was greatly reduced at thetime of reproducing the servo signal compared to the magnetic tapes ofComparative Examples 1 to 6.

In general, the squareness ratio is known as an index for a state of theferromagnetic powder present in the magnetic layer. However, as shown inTable 1, even in a case of the magnetic tapes having the same verticalsquareness ratios, ΔN's are different from each other (for example,Examples 3 and 4 and Comparative Example 5). The inventor has consideredthat this shows that ΔN is a value which is affected by other factors,in addition to the state of the ferromagnetic powder present in themagnetic layer.

An aspect of the invention is effective in a technical field of magnetictapes for high-density recording.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; a magnetic layer including ferromagnetic powder on thenon-magnetic support; and a non-magnetic layer including non-magneticpowder between the non-magnetic support and the magnetic layer, whereina total thickness of the magnetic tape is equal to or smaller than 5.30nm, a thickness of the non-magnetic layer is 0.10 nm to 1.50 nm, themagnetic layer has a servo pattern, a center line average surfaceroughness Ra measured on a surface of the magnetic layer is equal to orsmaller than 1.8 nm, and an absolute value ΔN of a difference between arefractive index Nxy of the magnetic layer, measured in an in-planedirection and a refractive index Nz of the magnetic layer, measured in athickness direction is 0.25 to 0.40.
 2. The magnetic tape according toclaim 1, wherein a difference Nxy−Nz between the refractive index Nxyand the refractive index Nz is 0.25 to 0.40.
 3. The magnetic tapeaccording to claim 1, wherein the total thickness of the magnetic tapeis 3.00 μm to 5.30 μm.
 4. The magnetic tape according to claim 1,further comprising: a back coating layer including non-magnetic powderon a surface of the non-magnetic support opposite to a surface providedwith the magnetic layer.
 5. The magnetic tape according to claim 1,wherein the center line average surface roughness Ra measured on thesurface of the magnetic layer is 1.2 nm to 1.8 nm.
 6. The magnetic tapeaccording to claim 1, wherein the servo pattern is a timing-based servopattern.
 7. The magnetic tape according to claim 1, wherein thethickness of the non-magnetic layer is 0.10 μm to 1.00 μm.
 8. Themagnetic tape according to claim 1, wherein the thickness of thenon-magnetic layer is 0.10 μm to 0.70 μm.
 9. The magnetic tape accordingto claim 1, wherein the thickness of the non-magnetic layer is 0.10 μmto 0.50 μm.
 10. A magnetic tape cartridge comprising: a magnetic tape,which comprises: a non-magnetic support; a magnetic layer includingferromagnetic powder on the non-magnetic support; and a non-magneticlayer including non-magnetic powder between the non-magnetic support andthe magnetic layer, wherein a total thickness of the magnetic tape isequal to or smaller than 5.30 μm, a thickness of the non-magnetic layeris 0.10 μm to 1.50 μm, the magnetic layer has a servo pattern, a centerline average surface roughness Ra measured on a surface of the magneticlayer is equal to or smaller than 1.8 nm, and an absolute value ΔN of adifference between a refractive index Nxy of the magnetic layer,measured in an in-plane direction and a refractive index Nz of themagnetic layer, measured in a thickness direction is 0.25 to 0.40. 11.The magnetic tape cartridge according to claim 10, wherein a differenceNxy−Nz between the refractive index Nxy and the refractive index Nz is0.25 to 0.40.
 12. The magnetic tape cartridge according to claim 10,wherein the magnetic tape further comprises: a back coating layerincluding non-magnetic powder and on a surface of the non-magneticsupport opposite to a surface provided with the magnetic layer.
 13. Themagnetic tape cartridge according to claim 10, wherein the thickness ofthe non-magnetic layer is 0.10 μm to 1.00 μm.
 14. The magnetic tapecartridge according to claim 10, wherein the thickness of thenon-magnetic layer is 0.10 μm to 0.70 μm.
 15. The magnetic tapecartridge according to claim 10, wherein the thickness of thenon-magnetic layer is 0.10 μm to 0.50 μm.
 16. A magnetic tape apparatuscomprising: a magnetic head; and a magnetic tape, which comprises: anon-magnetic support; a magnetic layer including ferromagnetic powderand on the non-magnetic support; and a non-magnetic layer includingnon-magnetic powder between the non-magnetic support and the magneticlayer, wherein a total thickness of the magnetic tape is equal to orsmaller than 5.30 μm, a thickness of the non-magnetic layer is 0.10 μmto 1.50 μm, the magnetic layer has a servo pattern, a center lineaverage surface roughness Ra measured on a surface of the magnetic layeris equal to or smaller than 1.8 nm, and an absolute value ΔN of adifference between a refractive index Nxy of the magnetic layer,measured in an in-plane direction and a refractive index Nz of themagnetic layer, measured in a thickness direction is 0.25 to 0.40. 17.The magnetic tape apparatus according to claim 16, wherein a differenceNxy−Nz between the refractive index Nxy and the refractive index Nz is0.25 to 0.40.
 18. The magnetic tape apparatus according to claim 16,wherein the thickness of the non-magnetic layer is 0.10 μm to 1.00 μm.19. The magnetic tape apparatus according to claim 16, wherein thethickness of the non-magnetic layer is 0.10 μm to 0.70 μm.
 20. Themagnetic tape apparatus according to claim 16, wherein the thickness ofthe non-magnetic layer is 0.10 μm to 0.50 μm.